297 t o 299 was held by the Division of Gas and Fuel Chemistry during the 706th M e e t i n g of the AMERICAN CHEMICAL SOCIETY a t Pittsburgh. Although this material has been published before in one form or another, w e present i t t o our readers as a compact summary of our knowledge of the primary liquefaction o f coal by hydrogenation. H. H. Storch, of the Pittsburgh Experiment Station, U. S. Bureau of Mines, was the leader o f the discussion; his photograph appears below on this page. It is under his direction that important work on the hydrogenation of coal has been going on in the bureau’s experimental plant. M. W. Kiebler, Jr., H. H. Lowry, and C. Howard, of Carnegie Tech’s Coal Research Laboratory, took part in the discussion, as did also G i l b e r t Thiessen, Koppers Company, and H. B. Charmbury, School o f M i n e r a l Industries, Pennsylvania State College. I n Britain the lack o f petroleum made the utilization o f coal resources a necessity years ago. A t the l e f t is a view o f liquid-phase coal hydrogenation converters at Billingham, England. NOWpublic opinion has been aroused i n this country to the extent that the U. S. House of Representatives recently passed a bill authorizing a number of experimentel plants to make syntheticgasoline from coal and other products.
H.
STORCH.The purpose of this meeting is to discuss some of the difficulties involved and to get suggestions for further research. I should like to limit the discussion to the chemistry of coal hydrogenation. That is, we should not at this time bring up the application of coal hydrogenation to industrial processes, With the added stipulation that welimit ourselves to the principal constituents of coal, we will avoid arguments about the hydrogmation of extraneous materials, such as fusain or opaque matter, which hydrogenate with considerable difficulty. \?‘e N ill try to discuss mainly the hydrogenation of vitrain or anthraxylon which is the principal component of most bituminous and subbituminous coals and lignite. The place to start is the phenomena involved in the action of solvents on coal. Whether or not you employ a solvent in coal hydrogenation work, shortly aftw the reaction commences H solvent is formed. In the work done at the Carnegie Institute of Technology, it was dcsirahle to iivoid a solvent in order t o identify the primary products and obtain them in uncontaminated form, free from extraneous solvent. Investigators there wed large quantities of catalyst i n order to aroid extrnbive repolymerization and rearrangements, which give materials that w e much more difficult to hydrogenate than the original coal. If siznblc amounts of catalyst arc used in the early stages of the waction, a solvent is formed from the hydrogenation of the coal.
The latter statement is based on work done in Canada by Warren and G i o r e , and in England in the Fuel Research Laboratory, where the effect of different amounts of catalyst was compared with that of a solvent plus small amounts of catalyst. Whatever the approach is, t h m , one of the first things that happens in any coal hydrogenation experiment is thesolution or depolymerization of the coal s u b s t a n c e . We would like first to talk about t h c nature of that process, what happens to the coal when it goes into solution Is it a simple affair like the solution of aome pure organiccompound in a good solvent, or is i t more complex” Evidence indiratrs
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M. W. Kiebler, Jr.
that it is decidedly more complex, that not much true solution is formed, and that even before a colloidal solution is obtained, certain things have to happento the coal which require considerable energy. Studies of temperature coefficients indicate that solvent action on coal involves an activation energy of 7 to 10 kg.-cal., and that the reaction is probably solvation or depolymerization of the coal substance. Will some of the men from Carnegie Tech briefly summarize the work done there on the solution of coal?
KIEBLER. My mind is not too fresh on all the details. This energy of activation, I think, is of interest. We have considcred both hydrogenation and solution of coal to involve depolymerization. The amount of material soluble in a given solvent and the yield of extract were correlated with various physical pioperties of the solvent; the best correlation was found with internal pressure. I n work done rather sketchily later, we attempted to use solvents a t varying temperatures but constant internal pressure, and evaluated this energy of depolymerization at around 10 kg.-cal. per "mole" of coal. We concluded from this study of correlation between internal pressure of solvent and yield of extract that the yield was made up of two parts-first the part due to the thermal depolymerization of coal, and secondly the part due to the solvent depolymerization of coal. This is borne out by the equations obtained, of the form: Y =A BPj = yield of extract where Y Pi = internal pressure of solvent A , B = constants depending on temperature
+
Constant A changes sign somewhere between 250" and 300" C. From this you can conclude that in that neighborhood the molecular weights of the products which result from thermal depolymerization of the coal reach a magnitude of about 350. We made a brief study of solvent mixtures, using phenol and Tetralin, and found that Pittsburgh seam coal, which is quite resistant to solution, was about 40% soluble in a 50-50 mixture of phenol and Tetralin a t 350" C. in 8 hours. This work was carried out in a pressure bomb. We conducted high-pressure Soxhlet extractions with benzene and three better solvents-aniline, Tetralin, and phenol. I n the case of Tetralin, extrapolation to infinite time of extraction gave yields better than 90%; benzene in infinite time, I believe, gave around 30%. The other two fell between. Aniline is not so good a solvent as phenol. We questioned the effect of the acidic or basic nature of the solvent on the solution of coal. From stepwise extraction of coal with aniline and with phenol in rotation, we concluded that the acid or the basic nature of the solvent was without effect. That is, neither one extracts specific constituents. This is certainly true within 1 or 2%. The products of extraction are difficult to handle, and it is easy to make small errors. They have a hahit of holding the solvent tenaciously so that i t is difficult to distill the solvent from the soluble constituents. We considered a large number of properties in this correlation of extract yield with physical properties. We handled about fifty-seven solvents statistically and showed that the best correlation was with the internal pressure of the solvent a t the temperature of extraction of solution. Correlation coefficients at 200' C. for that number of solvents were somewhere around 0.76. Next we took all the stable compounds a t 300" C., and again the
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correlation coefficient was about the same order of magnitude, around 0.7. Then we selected five or six solvents from that group and studied them a t 150' and 250" C. Since they were selected solvents, the correlation was somewhat higher; in some instances the coefficient was about 0.98. I think we definitely demonstrated the fact that there was a significant correlation. Surface tension correlated with yield of extract for certain solvents a t only one temperature, and we concluded that, in general, it was without statistical significance. Other physical properties are probably automatically taken into account in the correlation of yield with internal pressure. When deviations were plotted from the best line representing the relation of extract yield to internal pressure against various other physical properties, no orderly arrangement of points was lound.
STORCH. Did you relate test solvents t o the ease with which they could release hydrogen to the coal? KIEBLER. We assumed that the solvents used were stable, and they were tested for stability in a small pressure vessel. The indication we had of instability was rise of pressure with time at constant temperature and also the identification of decomposition products from the solvent. Tests were run in the presence of coal in the same type of reaction vessel used for the extractions. All of the group of solvents just discussed were stable throughout the reaction. I n the case of Tetralin, at higher temperature hydrogen was lost to the coal, and hydrogen balances of the coal products indicated that this was the case. After removal of the solvent from the soluble material, the balance (sum of weight of extract and residue material) was greater than the weight of the original coal. TTe assumed, therefore, that the excess was from the solvent. Hydrogen-Producing Equipment in the Experimental Coal Hydrogenation Plant at the U. S. Bureau of Mines' Pittsburgh Experiment Station
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INDUSTRIAL AND ENG INEERING CHEMISTRY
Some solvents are strongly polymerizing. Aniline, phenol, cresol, and o-phenylphenol have a tendency to combine with either the extract or the residue, and give balances which are above 100%. After a great deal of effort we did succeed in beating those down to an excess of about lo%, but it is not an easy job. The materials are black or dark brown amorphous solids. They are not tarlike, in the case of Pittsburgh seam coal. A MBMBER. What experimental procedure did you use?
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KIEBLER. 100 grams of solvent and 5 grams of coal were put into a bomb and heated for about 120 hours in duplicate. At the end of that time the contents of the bomb were poured or scrubbed out with a mixture of 50-50 ethanol-benzene, which is a good solvent for the particular type of compound found in the extract. Then the products were filtered through medium-porosity alundum thimbles and extracted for 120 hours a t atmospheric pressure in a Soxhlet type extractor with ethanol-benzene. The residue still in the thimble was set in the drying oven for a few hours, and then put into a vacuum desiccator and pumped for about 24 hours. The combined material containing the extract, original solvent, and ethanolbenzene was distilled on a water bath at atmospheric pressure, and then redistilled on a water bath a t several millimeters pressure t,o remove the solvent. I n the case of phenol and aniline we could pre:ipitate the extract with sodium hydroxide or hydrochloric acid, depending on the solvent. When we used hydrochloric acid, the extract material filtered off nicely. When we used sodium hydroxide about two or three out of ten extracts centrifuged excellently, but the rest refused to come down. We tried to keep all the extracts the same but still had trouble. The filtration required anywhere from a day to a couple of weeks for completion. This was the general procedure, Then the extracted material wm dissolved in ethyl ether, and a correlation was found between the amount of material insoluble in ether and the internal pressure of the solvent with which it was originally obtained. STORCR. The general picture from your work is that materials of high internal pressure and possibly of high polarity are good solvents. Highly polar compounds in general are good solvents?
b
a
KIEBLER. Polar compounds are good solvents with one exception-ethylene glycol, which has the highest internal pressure of all the compounds studied, and turned out to be one of the poorest solvents. Water has a high internal pressure too and is of no value whatsoever as a solvent for coal. From those two scattered points you might guess that the relation between yield of extract and internal pressure is not linear but may go through some sort of maximum. We plotted straight-line functions because we had insufficient data to indicate they should be otherwise.
STORCH.I n connection with the question asked about hydrogen transfer, the temperatures used in the solvent extraction work were so low that comparatively little hydrogenation actually occurs. There is some hydrogen transfer from Tetralin or similar types of hydrogen carriers, even a t relatively low temperatures, but it is not appreciable. LOWRY. At 300" C. with Tetralin we obtained balances, I believe, of over 125%. Whether it was a question of reaction between the hydrogen of the Tetralin and the coal or whether certain polymerizations of the Tetralin itseLf occurred was not established. The products were high in hydrogen compared to the original coal. KIEBLBR. They had a tendency to be asphaltlike, as I recall them. I n nearly all other cases the hydrogen balances ran slightly under 100%. We lost a small amount of hydrogen in almost every run. I n the case of Tetralin the Ioss was over 100%.
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LOWRY. There is one other point. As far as we could tell, the solutions were optically clear when they were hot, and they rapidly became colloidal and showed Brownian movement with the dark-field microscope. If the products were allowed to stand in the solvent, they gradually repolymerized and settled out on the bottom of the flask and on the walls. They were not stable compounds as they were taken out in the solvent. STORCH.But originally the solution was optically clear?
H. H. Lowry
LOWRY. Originally the solution was optically clear when it was taken out of the bomb and was still hot. Perhaps Dr. Howard will say something about the nature of the extracts from a study of their molecular weights, and then briefly review Biggs and Weiler's work. HOWARD.To elaborate on Dr. Lowry's point, we have good evidence that the material was molecularly dispersed in solution. These extracts are almost entirely free from ash; i t seems improbable, if it were a peptization process, that we would peptize merely an ash-free part of the coal. W e feel that they really were in molecular dispersion. Their properties in solution are of some interest. Materials extracted from coal by bensene, Tetralin, or some active solvent are incompletely soluble in less active solvents such as the petroleum hydrocarbons. If you take a benzene solution of the extract and add an excess of hexane or heptane, you precipitate the extract completely. These products are completely aoluble in benzene when fresh, and ebullioscopic measurements in this solvent indicate molecular weights in the range of 1000 to 1200. If similar measurements are made in other types of solvents, the apparent molecular weight varies greatly. With an active solvent such as phenol or catechol (strongly polar materials), the apparent kinetic unit has an average molecular weight of around 300. It is of interest also to point out that many of the other primary degradations of coal show molecular weights of this order of magnitude in phenolic solvents. We are not sure exactly what this means, but it seems to indicate that these primary products are held together by rather weak valence forces which are ruptured in active solvents. Association and dissociation seem to be characteristic features of the primary degradation products from coal.
STORCH.I am interested in the statement that these solutions are optically clear initially; it indicates that, if you have s o m e thing that will stabilize these initial depolymerization products of 'coal, such as a fairly high concentration of hydrogen or of hydrogen carriers, the coal hydrogenation can proceed from that point with the least difficulty. If you let such a solution stand-that is, if you try to separate the coal hydrogenation process into two stages (let us say first, the solution of the coal and subsequently the hydrogenation of the extract)-it is apparent that you may be a t a severe disadvantage. Some of your material may have repolymerized and formed compounds more stable and more difficult to hydrogenate than the original coal. HOWARD. We happened on catechol as a solvent for cryoscopia measurements more or less accidentally. Recently a good deal of information has been appearing in the literature concerning the inhibition of polymerization by such materials as catechol, and there may be some relation between the low molecular weights observed in this material and its inhibiting action.
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STORCH. Some work was done in Germany by two chemists, Pott and Broche, on the solvent extracts of coal with mixtures of Tetralin, phenol, and naphthalene. They found that for each coal, or rather each type of coal, there was an optimum trmperature at which they got fairly high concentrations of coal in solution. Up to 80 or 90% of the coal dissolved in approximately an equal weight of the solvent For bituminous coal. those optimum temperatures were in the range 390" to 410" C. These workers also found that, if they allowed the solution to remain at these optimum temperatures beyond a aertain maximum time, they obtained lower yields; that is, reprecipitation occurred. There was a repolymerization of materials which werc initially in solution; owing to the fact that there was nothing to stabilize them and keep them in solution, they reacted with onel another and formed more stable polymers. Hence the yield o f extract was lower.
FRANRE. There was one more point-that made by Ilr. Lowry about Biggs' work. It indicated, as I recall, that the extract and the residue were similar in structure, and it was just a matter of depolymerization of the coal by the solvents This was based on chemical rather than physical evidence. I think that point is reasonably important to mention at this time. STORCH. That was work done on the benzene extraction. The extract and residue were found to be very similar in genrral properties. Upon hydrogenation of both of them, similar compounds were obtained; in general, studies indicated that the lowest molecular weight in catechol was the same for both. LOWRY. The distribution of products by molecular weight were also about the same for extract and for residue. STORCH They were not of the same molecular \wight, but the distribution was the same. LOWRY. The distribution by molecular weight was about the same. Hydrogenation was at 375' C., using Adkins' catalyst. The products were fractionated, and the number of rings was determined by something like Waterman analysis. KINKEY. Going back a little bit, what do you mean by an activation energy of 10 or 12 kg.-cal. per mole of coal? Tc; that on the basis of weight? STORCH. I n making this calculation of activation energy, there is no assumption about molecular weight. It is a sort of statistical average, and the result applies in a statistical manner. That is, the average molecular species present requires approximately that much energy per molecule to go into solution. There i s no assumption, and there is no necessity for any assumption, about molecular weight. The 10 kg.-cal. is significant only because it indicates that the bonds which are broken are weak, that they may be coordination valences of one kind or another, and that they are not primarily valences. It merely gives an idea o f the character of the broken valences.
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coal tar or coal-hydrogenation heavy oil. But they would certainly be superior solvents. The crudest way of putting it is that like dissolves like. Aromatics are much better solvents than aliphatics or olefins. The more condensed the ring structure of the solvent, the higher yield of extract obtained. I think hydrogenation products would be far better than any of the solvents we used. LOWRY. One indicat'ion that they are excellent solvents is that primary liquefaction carried out at relatively low temperatures can almost completely liquefy the coal. And there is no solvent which, under those conditions, would give complete solution of the coal or tjhesame degree of solution.
A MEMBER. Using aromatic solvents, was it found tha.t t.he higher-molecular-weight solvents were more effective than those of lower molecular weight? Putting it crudely, would a highboiling creosote oil be a much better solvent than benzene oil? KIEBLER. If you put it that way, I think it would be; but there are certainly other considerations. The polar nat'ure of the solvent would be one. A MEMBER. Did you use pure aromatic hydrocarbons? KIEBISR. We used as pure compounds as we could conveniently obtain. STORCH. I 'should now like to carry the discussion a little further. The primary products of coal hydrogenation, materish that you can isolate early in the reaction, are pitches. That is, they are high-molecular-weight materials which are difficult to (reat chemically. Perhaps the most interesting fact about these pitches is that regardless of the coal used, provided you start with a coal of no higher rank than high-volatile bituminous, you finish with approximately 3% oxygen in these pitches. In other words, during the early stages of the coal hydrogenation, you get rid of the oxygen rather rapidly, chiefly as water, secondarily as carbon dioxide; you obtain a residual material which, regardless of the coal used, contains approximately 3% oxygen. I do not know whether I am overemphasizing that point. I suspect it has some significance. It should be correlated, for example, with the fact that if you carbonize peat at a low temperature (around 400' C.) and then hydrogenate the residue (char), it behaves in general like a bituminous coal. Therefore I am inclined to believe that this figure of approximately 3% oxygen represents a certain proportion of very stable oxygen groups, which are either formed during the earlier portion of the hydrogenation and pyrolysis, or are present initially in coals of all ranks to about the same extent. At present I see no evidence for deciding between those two possibilities.
HOWARD. Biggs did a little work on the nuture of oxygen in coal, working with a Pittsburgh seam bituminous coal and Adkins' catalyst. At 350' C. the elimination of oxyUNDERWOOD AND UNDERWOO0 A MEMB~R.You ordinarilyuse a product gen was rather incomplete, more was removed William C. Howard of hydrogenation to disperse the coal for subat 375' C. and, under our conditions, fairly comsequent hydrogenation. How does that Dlete elimination took d a c e a t 400" C. The fit into the list of solvents tried? Is it a good solvent or a bad products formed at these different temperatures were examined solvent, or was it tried? for hydroxyl groups, and the results indicated that the stable oxygen originally present in coal is first transformed to hydroxyl, LOWRY. Probably the best, but those materials were not perhaps by the opening of a cyclic oxygen ring. Later these tried in our studies. hydroxyls are eliminated by further hydrogenolysis reaction;. KIEBLER.One of the reasons they were not tried was that STORCH. You would, then, be inclined to say that the more we were interested in the relation of the effectiveness of a solvent stable oxygen is derived from the more reactive groups initially to its physical constants, and there would have been real difficulty in evaluating the physical constants for mixtures such as present in the coal, on the basis of your evidence.
April, 1944
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the aromaticity of the products increases with increasing rank LOWRY. This may or may not be pertinent to the discussion; and that you do get more of the aliphatic hydrocarbons from but in the analyses of cokes prepared by the Bureau of Mines coals of lower rank. But the data are by no means conclusive or from a large number of coals at different temperatures, there is good evidence that the oxygen (say in the 500' C. cokes) is quantitative, and show only a general trend. dependent upon the oxygen content of the original coal. I n other I n addition to these hydrocarbons, you also obtain materials words, some of the oxygen of coal remains in the 500" coke. At generally known as tar acids and bases. The tar acids, which 600" C. the dependence is much less, and a t 700" and higher temare mixtures of various types of aromatic hydroxyl compounds, peratures there is no dependence. contain materials that boil all the way u p through 400" to 450' C. They are very comA high-sulfur coal-for example with 4% sulplex, difficult to separate, and extremely refur-apparently releases its oxygen much more active. Dr. Thiessen will tell us about the easily than a low-sulfur coal. The sulfur is held nature of some of these tar acids obtained from strongly by the coal and is completely fixed at one of our low-rank coals. 500" C.; it is not driven off up to 1100' in the same proportion as is the volatile THIESSEN.Some months ago, L. L. Hirst reported at a divisional meeting of the Pittsmatter. burgh Section, AMERICAN CHEMICAL SOCIETY, There is a question as to the similarity in type on some of the work a t the Bureau of Mines. of linkage, particularly with the organic sulfur He mentioned the tar acids produced by the and the oxygen in the coal. I do not know hydrogenation of coal and showed their diswhether there is any significance to that point tillation range. The question was asked as to or whether these high-sulfur coals would behave the make-up of those acids, as to their content differently on hydrogenation from a low-sulfur of phenol and of 0-, m-, and p-cresols, and parcoal. The difference between oxygen and sulticularly as to the m- and p-cresol contents fur in hydrogenation reactions is not known, which vary with the coal and with the way but there is some evidence of interchangeability the tar acids are produced. The Bureau of Gilbert Thiersen of oxygen and sulfur in the coal structure Mines had no data to answer these questions itself. but gave us the sample for an investigation STORCH.I think the fact that you got an amount of oxygen of our own. It has taken us quite a time, since we have worked ih in your cokes approximately proportional to the amount of in along with other work, and it has not been easy. oxygen in the original coal can be explained on the basis that These acids partake of many of the properties of low-temyou do obtain these more stable oxygen linkages as a result of perature tar acids. Those of you who have worked with them pyrolysis of the initial group. And since you have more present know that they tend to be more complex than tar acids from highinitially, you would expect, from those coals in which more oxygen temperature tar. They tend to be more highly substituted or is present, to have more in the coke, even at reasonably high have a greater variety of substituents, and possibly to contain temperatures. The evidence that you and Dr. Howard have other materials, such as poly- or dihydroxy and sulfur compounds. presented indicates that the more stable oxygen linkages, which We have not determined the sulfur content, but the characterhydrogenate with difficulty, are probably formed by pyrolysis, istic stench of some of the fractions indicates the presence of sulfur hydrogenolysis, or both, of the more reactive oxygen groups which compounds. are initially present in the coal. David Craig and Hugh Rodman carried out the distillation, I n this connection, although considerable carbon dioxide is later Dr. O'Brochta and Rodman did another distillation, and evolved in the hydrogenation of subbituminous coals and lignites, C. A. Johnson ran some of the tests. I n a preliminary distillation the proportion of the total oxygen that appears as carbon dioxide with a flask and condenser, which was a precaution to protect our is not large. It is of the order of 10 to 20%. Therefore you still large fractionating column, they found that the coal contained have a great deal of oxygen appearing as water in the hydrogenaconsiderable amounts of a very pitchy, tarry residue. We distion reaction. tilled about 2.5 liters of the tar acids, which were from Velva Now let us carry the hydrogenation reaction a step further. lignite. They were first roughly distilled to leave behind apSince we know so little about these pitches, we will leave that to proximately 7% pitch, and yielded 4.2% water and 86.5% disfurther research. It is a little difficult now even to indicate a tillate. There was 1.3% loss which may be gas. This material line of attack on these primary products. My only suggestion was then redistilled in a Fenske type of fractionating column, is a study of molecular weight with the ultracentrifuge and an approximately 100 inches high with 60, 70, 75, or 100 theoretical attempt to correlate that molecular weight with hydrogenation plates, depending on how you use it. conditions. But even such an elaborate study will not give any This distillation curve obtained is shown in Figure 1. It is real information about the structure of these pitches. It will not the sharp curve usually obtained when distilling coke-oven provide only an idea of the kinetics of the breakdown of the coal tar acids. Ordinarily we can cut phenol, o-cresol, and a m-cresolmolecule. These pitches which are in solution in the solvent, p-cresol fraction easily. As an example, the original estimate usually obtained from the coal itself in hydrogenation reactions, was that this material contained 10% phenol. The phenolsubsequently form distillable products. As soon as we can distill o-cresol fraction was composited on the basis of the curve; then these products, we can attempt to obtain reasonably pure comthe individual tar acids were determined chemically, phenol by pounds, try to determine the molecular weight accurately, and the Chapin method and o-cresol by the cineole method. The learn something about structure. m-cresol content of the meta-para fraction, which was to be free of The work carried out in various laboratories in the United phenol and of the xylenols, was determined by the nitration States and abroad indicates definitely that these distillable method-that is, by the method of the British Tar Producers' products are polycyclic. Usually they are aromatic hydfoAssociation. All of these methods are standard. carbons with two, three, or more rings and with side chains Along with the analysis of this material, you will be interested attached. Experiments at the Carnegie Institute of Technology in an analysis of normal tar acids for comparison. This comparion coals of several ranks indicate comparatively little difference son cannot be made directly because tar acids are extracted comin the general nature of these distillable products as a function of mercially from a tar acid oil which has a definite boiling range. rank. Work done at the Bureau of Mines on coals of different This material obtained from the Bureau of Mines did not have ranks in the continuous coal hydrogenation plant, indicates that the usual boiling range but contained much higher-boiling ma-
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Figure 1. Distillation through 100-Inch Packed Precision Fractionating Column of Tar Acids M a d e b y Liquid-Phase Hydrogenation of Velva Lignite at the Bureau OF Mines
terials. For example, a commercial crude cresylic acid includes only phenols, cresols, and xylenols. I n the Bureau of Mines sample the phenols, 0-,p-, and mcresols, and xylenols added up to only 60%. In other words, other materials were present which were higher boiling than commercial tar acid, so that all figures must be divided by 0.6 to compare the bureau’s sample with industrial tar acids. Commercially, the ratio of m to p-cresol is important. The ratio of phenol to total cresols is of interest. Analysis of the bureau’s sample was complicated by the presence of constituents which are normally absent in commercial tar acids. The lowest boiling material, apparently lower boiling than phenol, was not water or hydrocarbons. We think it was thiophenol, judging by the first plateau in the curve and the stench of sulfur compounds in this fraction. The analysis of the various tar acids follows: Pheno rCresol m-Cresol
%%Zl1 Meta/(mets. Pheno1:oresol
+ para)
Hydrogenation
Hydrogenation X 1.734
7.4% 4.3 7.3 9.7 29 57.7 43% 1:3
12.8% 7.5 12.7 16.8 50.2 100.0 43 % 1:3
Coke Oven
;:%
17 13 21 100
57%
1:lV:
Using the equal area method of cutting on the curve, the first estimate was 10% phenol. Actual chemical determination gave 7.4, which indicates how ragged the cume was. The figures in the last line are fist-order approximations. The amount of xylenol in the phenol to xylenol fraction of these tar acids is 50.2%, compared to 21y0in the coke-oven tar acid. I n addition, the total hydrogenation tar acids contain 42.3% of material boiling above xylenol. The m-cresol content of the meta-para fraction was 43%; the coke-oven tar acids contained 57% m-cresol and 43% p-cresol. The ratio of the phenol t o the sum of the 0-, m-, and p-cresols was about 1to 3, and in the cokeoven tar acid, was about 1 to 1.3; the coke-oven tar acida were exposed to higher temperatures and had been cracked down more than the hydrogenation tar acids. STORCH. This large proportFn of xylenol seems to be characteristic of practically all the coals that we have hydrogenated. We get much more of the xylenol than we do of the cresol and phenol fractions. We know virtually nothing about the higher tar acids except that they are a nuisance if we have to work with them and separate them from other compounds. In the work a t Carnegie Tech, one of the prime requisites for efficient separation is t o get rid of some of the higher-boiling tar acids because of their interference with isolation of other groups of materials. LOWRY. Does Dr. Thiessen have similar data on low-temperature tar? THIESSEN. We do, but I did not assemble them; they were assembled from scratch paper this morning. Perhaps Mr. Rhodes or Dr. O’Brochta remembers the composition of lowtemperature tar acids.
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RHODES.The data on low-temperature tar acids are similar to those for the hydrogenation tar acids. There are large amounta of xylenol and high-boiling acids. Do you have any figure on the ratio of meta to THIESSEN. para? RHODES.Not with me. THIESSEN. That ratio is rather interesting, because there is a distinct difference in the value of m- and p-cresols. A MEMBER. Are those coke-oven figures on Velva lignite? THIESSEN.No. This is typical coke-oven tar acid. From a commercial standpoint the present market is based on this type of material and is the basis for comparing the data on the hydrogenation of tar acids. A MEMBER. Those are per cent on basis of total tar acid? THIESSEN.They all go back to total tar acid. THEMEMBER. How would they compare on the basis of the original coal? THIESSEN.I do not know what the yield was. THEMEMBER. What are the yields of tar acids as compared to the weight of total coal, approximately? STORCH. I n hydrogenation you obtain about ten times the yield that you do in carbonization; roughly, from 10 to 15% of the ash- and moisture-free coal will be obtained as tar acids. T H MEMBER. ~ Do you get higher yields from lignites than from other coal? STORCE. Yea. We obtain appreciably higher yields from subbituminous coals and lignite. We gave Dr. Thiessen tar acids from lignite because we happened to have a large sample available. We have always been more curious about the composition of those tar acids than of the tar acids from some of the higherrank coals. A MEMBER. Was there any preliminary treatment of these tar acids? THIESSEN.As I mentioned, the investigators gave the material a kick-over distillation to get rid of the pitchy residue. Toward the end of the distillation the acids tend to decompose, and the pitch heads up into the bottom of the column packing rings. The acids were given a kick-over distillation to remove some of the very high boilers. The results were all calculated back to the original sample. 9
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2. Rate of Hydrogen Absorption in Presence OF Tin
Sulfide Catalyst Point of zero absorption is shifted on ordinate to mako each curve distinct.
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. April, 1944
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THEMEMBER. I mean, you did not reprecipitate?
THIESSEN. Oh, no. A MEMBER. About what would be the 90% distillation point on the oil from which these Velva lignite tar acids were obtained? Was it a moderately high-boiling oil? STORCH. The 90% point would be somewhere around 325” C. A MEMBER. Has any work been done on the still higher-boiling oils from that type of tar? STORCH. Practically none.
I think we should say a word about the neutral materials.
c
We do not know much more abqut them than we do about the tar acids. We know in a general way that we obtain alkyl naphthalenes and benzene homologs, with side chains of varying length; that we get condensed carbocyclic structures similar to those obtained in carbonization of the same coal; and that compounds such as anthracene, chrysene, phenanthrene, and carbazole have been isolated from both coal hydrogenation and coal carbonization products. A promising attack is being made on the nature of these materials a t Carnegie Tech, where some of the primary products have been separated by adsorption on a column of alumina, and various bands are being studied by competent men to get some idea of the structure of these materials. Also, the kind of work being done a t Pennsylvania State College on pyrolysis of pure materials will give us an inkling as to whether the behavior in coal hydrogenation reactions is similar t o that type of reaction in pure compounds. I want to come back particularly to the matter of carbon dioxide evolution. It has been correlated with several other factors. For example, if you plot the solubility of coals in alkali (say potassium hydroxide), the curve exhibits a break a t about 82% carbon. That is, you get a curve that changes slope rapidly a t about 80% carbon and becomes almost parallel to the solubility axis a t about 82% carbon. Coals become increasingly soluble with lower carbon content. If you plot the amount of carbon dioxide evolved upon hydrogenation, a similar break occurs a t about the same point. The carbon content on this curve is plotted along the X axis and the yield of carbon dioxide along the Y axis; as the carbon content decreases, the carbon dioxide yield increases slowly and then jumps around 82%. There is a marked increase and subsequently a flattening off. Another thing to include in this discussion is a plot of the ratio of oxygen atoms in coal against the carbon content. These data are based on Hickling’s analyses of about 1200 coals, and again there is a rather steep increaae in the general region of 80%. These three curves indicate that there is a definite correlation between the chemical changes which accompany metamorphism and the behavior of the coal as regards solubility in alkali and hydrogenation. I n a general way this correlation means that you have larger amounts of carboxyl groups in the lower-rank coals, and that the carbon dioxide is produced by the pyrolysis of those carboxyl groups. HOWARD.I n that connection the presence of carboxyl and hydroxyl groups in bituminous coals has been mentioned several times. As far as I know, there is little evidence for other than negligible amounts of either of those groups in bituminous coals of the rank of the Pittsburgh seam. I n the Coal Research Laboratory a t Carnegie Tech some measurements of alkali solubility were made by reacting two bituminous coals a t elevated temperatures with aqueous alkali. I n Pittsburgh seam coal we discovered negligible amounts of carboxylic material. I n Illinois coal, on the other hand, we recovered a considerable amount of carboxylic material. These facts seem to correlate with your observations on the amount of carbon dioxide recovered after hydrogenation.
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CHARMBURY.Y o u m e n tioned that the stable oxygen results from the pyrolysis of the more resistant acid oxygen groups. Would you care to mention any of the active groups that might be present? STORCH. In general, what by active groups are any aliphatic groups or polyhydroxyphenolic materials containing oxygen. By stable oxygen I have in mind particularly the aromatic ethers, such as diphenyl or diphenylene oxH. B. Charmbury ides. The evidence now is that you start with this lirst group which I call “active” and from it obtain a larger amount of the second g r o w which I call “more refractory”. They do not hydrogenate so easily.
I mean
Do you think that the so-called coat molecules CHARMBURY. are tied together by these oxygen linkages? STOROH. I am inclined to believe so, but there is little evidence. The idea was first put forth, I believe, by Erasmus, baaed on no evidence except general intuition. He postulated that the coal molecule was a condensation product of groups that contained about ten carbon atoms, and that these groups were hooked or linked together by oxygen atoms. That picture, in a general way, seems correct. Possibly the CIOfigure is a little low. I am inclined t o believe it is considerably higher than that, possibly as high as C I ~or CIS. Furthermore, I think that the groups or the units of this condensed structure are not uniform or all the same. You might have three or four different types of groups. I do not believe that the oxygen linkages are necessarily uniform and the same; you may have three or four types. Also, I am inclined to believe that coal is not a linear polymer like some of the synthetic polymers. I think coal has a certain number of cross linkages, so that it is partially a three-dimensional polymer; this would account for some of its refractory properties. It is somewhat brittle but will flow under high pressure. It is plastic; therefore it is not highly three-dimensional but only partially so.
I might mention one thing in connection with CHARMBURY. hydrogenolysis of pure compounds. The British Fuel Research Board has hydrogenated diphenylene oxygen, or diphenyl ether, and obtained little oxygen removal in the first stages. That is, the compound split with the formation of phenol, which later did hydrogenate and the oxygen was pulled out of it. But that fact ties in with what you just suggested. STORCH. I should like to conclude the discussion by a brief review of some of the results of the Bureau of Mines’ work on coal hydrogenation reactions, with which I am more immediately familiar. First, I should like to correct a statement made by C. C. Wright, or rather a statement made in our earlier papers which Wright quoted-namely, that the amount of hydrogen adsorption was a straight-line function of time. Subsequent work has indicated that it is not so simple as we first thought. I n Figure 2 the hydrogen absorbed in per cent of the moistureand ash-free coal is plotted against the time in hours. These experiments were carried out in a rotating autocalve, the hydrogen was put in initially a t 1000 pounds per square inch pressure, and a t the end of a specified period the hydrogen was renewed. If we were running a 6-hour test we renewed the hydrogen a t the end of 3 hours; that is, we blew down the bomb and put in the
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hydrogen again a t 1000 pounds pressure. So while we did not have absolutely constant partial pressure of hydrogen, vie approximated it. Although Figure 2 shows some reasonably straight lines for the low temperatures, 300-375" C., there is marked curvature a t the temperatures above 375" C. for the first hour of reaction, although the curves flatten out to approximately straight lines later. LOWRY. What are the temperatures? STORCH. They range from 250" up to 430" C. We are plotting the hydrogen absorbed against time, and each curve represents work at a different temperature. The lowest line is 250' C., and then we go to 280,310, 355, 375, 385, 400, 415, and 435' C. Pittsburgh seam coal was used in the presence of the hydrogen carrier-that is, in the presence of Tetralin and of 1 70 tin sulfide. CHARMBURY. Do you think that the first portion of the curve, during the first 3 hours we will say, may be due to t,he taking up of the hydrogen in the carrier? STORCH. There is some of that effect, of course, but I do not believe that explains these rises. We have made as careful an analysis of these curves as we can. We are particularly interested in their slopes, which give us an idea of the temperature coefficient. The temperature coefficient in this region below 300' C. indicates definitely that we are dealing with a diffusion reaction. There are only two possibilities: It is either the diffusion of hydrogen through a a m on the catalyst surface, or it is the diffusion of dissolved coal away from the original coal particle involved. Above 375' C. at very low reaction times the curves indicate a high energy of activation of about 60 kg.-cal. The curves for degree of liquefaction and for oxygen removal are similar. Those initial rises appear to be largely due to the fact that the slow step is the thermal decomposition of the coal. Beyond the first hour of reaction the activation energies at longer times drop off rapidly, and there appears to be a series of successive reactions. We cannot disentangle them on the basis of these studies. We are inclined to guess that,, subsequent to the thermal decomposition of the coal, the fragments are stabilized fairly rapidly by reaction with hydrogen carriers, and that as far as oxygen elimination is concerned, the slow step is probably the reaction of the hydrogen carrier with one of these stabilized groups. We have evidence to indicate that it is not a catalytic reaction, that the chief function of the catalyst is the regeneration of these active hydroaromatic carriers, and that the chief function of hydrogen pressure in primary liquefaction appears to be twofold: (1) the regeneration of active hydrogen carriers, and (2) the orienting effect high hydrogen pressure has on the pyrolysis of certain groups in the coal. As Dr. Charmbury's results indicate, with high hydrogen pressure there is not so much dealkylation of side chains as there is at lower hydrogen pressures.
A MEMBER. Are all the data that you h w e given so far for high-volatile lower-rank coals?
STORCH.Yes. THEMEMBER. Have you done any work on the higher-rank medium-volatile and low-volatile coals in the continuous plant?
STORCH.We have made one investigation on Upper Freeport coal which I believe is close to a medium-volatile coal. It is not so satisfactory as some of the lower-rank coals, but it could be hydrogenated. We suspect that it would give considerably more difficulty in a large plant than the Pittsburgh seam coal, largely because of asphaltic compounds which are built up in the recycle. THE MEMBER. But you have not worked with any of the true medium-volatile or low-volatile coals? I wondered because
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I understand some one from the Bureau of Mines gave a talk recently on coal hydrogenation a t Barnsboro, and the statement came back to me fhat the medium-volatile coals were the best for hydrogenation of any that you had tried. STORCH.The statement was either incorrect or misinterpreted. Our general conclusion is that the best coals for hydrogenation in the United States today are probably subbituminous, containing comparatively low ash and low opaque matter.
A MEMBER. Some of those medium-volatile coals have low opaque content, but they are still a little high rank. STORCH. When you get beyond 83 or 8401, carbon, thls asphaltlike material building up in the recycle is bound to give considerable difficulty on a large scale. A MEMBER. Do those "best" materials exclude or include the lignites? STORCH. They include the lignites, with the reservation that many of our lignites are high in ash and the ash of most coals has an undesirable catalytic action. That action is largeIy condensing in nature. Unless it is counteracted by the addition of larger amounts of expensive catalyst, the hydrogenation of highash lignites will cause considerable difficulty, aside from the matter of having an inert diluent (an inorganic ash) in the high-pressure system. Therefore low-ash lignites, or lignites that can be washed to a low ash, are just as good as some of these low-ash subbituminous coals, such as the Monarch bed in Wyoming.
THEMEMBER. Would lignite tars be all right? STORCH. Any tars are easily hydrogenated, compared to coal. A MEMBER. Are the coals that are hydrogenated in Germany washed before they go into their system? STORCII. I do not think so. The ash of the brown coal IS largely inherent, tied up with the coal structure, and almost impossible to separate by any simple procedure. A MEMBER. Js i t your idea that the Germans have used th:tt coal bacause it is available and not because it is the best? A MEMBER. Because it works best. They separate the abh in the first phase and then carry out second-stage hydrogenation. QTORCH. I should like to point out that coal hydrogenation has by no means been perfected. I think the Germans as well a4 the British are laboring under certain difficulties which have not been solved because industrial development has been pushed so rapidly that it has far outstripped the supply of fundamental information necessary for a rational development of the best type of hydrogenation process.
A MEMBER. What type of coal are the British handling now? STORCH. They are using a bituminour coal (Durham bed coal, from Yorkshire) which is very similar to the Pittsburgh seam coal.
THE MEMBER. It is different irom anything in Germany? STORCH. Yes. Their problems mere much more severe than the Germans', and they got comparatively little help from the Germans. Each coal is a different chemical individual in hydrogenation.
THE MEMBER. There i s no subbituminous coal in England available for that type of work? STORCH. No, there is not. Please do not misunderstand me. 1 do not say that you cannot hydrogenate lignites containing appreciable quantities of ash. You can. Rut if you have a subbituminous coal of low ash and low opaque matter, I would prefer it. If there are no further questions, we will adjourn
